backend-drm: fix issues in libbacklight

The value of brightness node in backlight sys file may end with '\n', which is not accepted by safe_strtoint.

Another issue is that buffer pointer is damaged by the read call, so it will be dangerous to free it if the following asprintf fails.

https://github.com/torvalds/linux/blob/master/drivers/video/backlight/backlight.c#L227

added DRM/KMS backend label

@xndcn Is there a particular platform I can test this on to validate it?

changed target branch from master to main

Just stumbled onto this issue today, came up with the same fix 😄 . Would appreciate it, if this could be merged. @cphealy on any hardware that supports brightness control this should be visible. After the fix is applied the logs will show something like: Initialized backlight for head 'eDP-1', device /sys/class/backlight/acpi_video0 and the display should be dimmable (with the correct access rights). It seems like this bug was introduced in 2016 with the commit 913d7c15.

@soerenmeier for this to be merged we need a s-o (https://gitlab.freedesktop.org/wayland/weston/-/blob/main/CONTRIBUTING.md#L99), to comply with https://gitlab.freedesktop.org/wayland/weston/-/blob/main/DCO-1.1.txt

Either @xndcn does that when re-basing, or you could open a new gitlab MR, and close this one.

Or potentially, replacing (all) asprintfs() with our str_printf helper.

mentioned in commit soerenmeier/weston@cf337923

mentioned in commit soerenmeier/weston@e506ec35

mentioned in commit soerenmeier/weston@009a2a3a

mentioned in merge request !818 (merged)

closed

@soerenmeier I have forgotten the context about this commit since it was submited one year ago, so you are welcome to open a new MR.

mentioned in commit soerenmeier/weston@edef8746

mentioned in commit soerenmeier/weston@c983fc09